In the gas production industry, there is a need to efficiently separate oxygen from oxygen-containing streams at ambient or sub-ambient temperatures.
In cryogenic air separation, nitrogen (N2), oxygen (O2), and argon (Ar) are separated based on their boiling points and relative volatilities. A first cryogenic column is responsible for a rough separation of the three main components of air: nitrogen (78%); oxygen (21%); and argon (1%). A side stream is removed and sent to a second column known as the side arm column or crude argon column. This stream is called “crude” because it exits this side arm column at only about 95% argon. The conventional methods of further purifying this crude argon are limited to: “Deoxo” purification; getter bed technologies; and additional distillation. The Deoxo process reacts controlled amounts of hydrogen with the oxygen in the argon stream to remove the oxygen. Because the reaction of hydrogen and oxygen generates significant heat, this process can be dangerous if not controlled properly. Getter beds only function at lower oxygen concentrations by reacting oxygen with copper catalyst to form copper oxide. When high purity argon is desired, a third distillation column can be used to concentrate it. Unfortunately, these distillation columns require upwards of 200 stages due to the similarity in boiling points of oxygen and argon and are less economical than is desired.
To achieve a kinetic separation of O2 from either N2 or Ar by an adsorption mechanism, an adsorbent structure must be developed with very specific pore dimensions. Although the Lennard-Jones 6-12 kinetic diameter of Ar (3.40 Å) is smaller than that of O2 (3.46 Å), O2 is not a spherical molecule and has a minimum molecular dimension that could be exploited. Adsorption mechanisms suggest that the minimum molecular dimension is the limiting factor for kinetic exclusion. With the proper orientation, O2 should diffuse into a pore with an effective diameter of 2.8 Å. Ar, a spherical molecule, will have a constant diameter of 3.4 Å. This 0.6 Å difference in diameters is the sensitivity that an O2 selective adsorbent must demonstrate. With such an adsorbent, a process could be contrived that purifies crude argon from the cryogenic air separation process in a safer and more economical manner and purifies N2 from O2 much more rapidly and efficiently.
Carbon molecular sieves have been developed that selectively adsorb O2 over N2 based on the smaller kinetic diameter of the O2 molecule, see e.g. Yang, R. T. (Gas Separation by Adsorption Processes, Butterworths, Boston, 1987). More recently, in S. U. Rege and R. T. Yang (Adsorption, 2000, Vol. 6, 15-22), and U.S. Pat. No. 7,501,009 to Graham, et al., carbon molecular sieves are proposed for removal of O2 from Ar by kinetic separation.
Nevertheless, use of carbon molecular sieves for the purification of crude N2 or Ar presents a number of problems, including, but not limited to, a potential for combustion if returned to the cryogenic distillation tower, variable pore size distribution, and other drawbacks. Zeolites are porous aluminosilicates, which are non-combustible and contain well controlled pore sizes based on their highly crystalline structure. As a result, they may overcome many of these drawbacks.
A chabazite-type zeolite has a three-dimensional pore structure containing channels with 8-membered oxygen ring openings of 3.8 angstroms. This ring diameter is close to the target pore dimension discussed for the kinetic separation of O2 from Ar and N2 vide supra. Naturally occurring chabazite-type zeolites are known, which have a variety of compositions and cations to balance the AlO2 unit charges of the aluminosilicate. Typical natural chabazite compositions are Mx(Si25.5Al10.5O72), where M is a mixture of cations and x is chosen to balance the charge of the AlO2 groups. Synthetic chabazites have been prepared by several methods including crystallization from Y-type zeolite as a raw material under hydrothermal conditions.
Chabazite zeolites are often described in terms of the ratio of Si to Al atoms in the framework of the zeolite composition. Hereafter, ratios with respect to silicon and aluminum atoms refer to atoms in the zeolite framework. Chabazite zeolites are often referred to in shorthand. For example, chabazite is often abbreviated as CHA. Hereafter a chabazite composition with a specific Si/Al ratio will be defined by specifying the Si/Al ratio in parenthesis. For example chabazite with Si/Al of 1.6 will be specified by CHA (1.6).
Natural chabazites typically have Si/Al ratios ranging from 2.2 to 3.5. Compositions isostructural to chabazite (CHA) with an Si/Al ratio as low as 1.4 and containing iron in their zeolite framework have been observed in nature (E. Passaglia in Natural Zeolites: Occurrence, Properties, Use, Pergamom Press, 1978, pp. 45-52.) Large quantities of pure natural chabazites are not readily available.
What appear to be single phase, synthetic CHA compositions with Si/Al from 1 to 1.5 have been described by Barrer in two publications. In the first, R. M. Barrer and J. W. Baynham (J. of the Chemical Society, 1956, 2892-2903) report compositions prepared by direct synthesis with Si/Al ranging from 1.15 to 1.36 and >1.96. No specific synthesis route was included and only limited analytical data was reported for these compositions. CHA compositions isolated with only a single cation type were reported for compositions with Si/Al<1.96. Chabazite compositions with mixed Na and Ca cation substitution were reported for compositions based on natural chabazite with Si/Al=2.5.
Low temperature adsorption data reported in this publication and collected at about −182° C. showed that, based on the slow uptake rates, O2 and Ar are both effectively excluded by CHA structures with Si/Al of 1.36 or less, regardless of cation substitution. This suggests that these compositions would be unsuitable for separating O2 from Ar.
In the second publication, R. M. Barrer and D. E. Mainwaring (J. of the Chemical Society, Dalton Transactions, 1972, 1254-1259) reported CHA compositions with Si/Al ranging from 1.07 to 1.5 and CHA with Si/Al=2.25. The compositions were prepared from KOH, metakaolinite, and amorphous silica gel. Even though the reported chabazites were prepared from varying gel compositions with SiO2 to Al2O3 ratios ranging from 2 to 10, no chabazites with Si/Al from 1.5 to 2.25 were observed. This suggests these are not stable compositions under the conditions described by Barrer. Materials with Si/Al=1 and 2.25 were characterized by X-ray diffraction, but no analytical data were reported on compositions with Si/Al>1 and <1.5. Again, no mixed cation forms of these CHA compositions were reported. Data from adsorption studies on Li, Na, K, Ca, and H forms of CHA (Si/Al=2.25) were reported. Uptake of O2 was observed at −195° C. in all of the cation forms with the exception of potassium. KCHA (2.25) completely excluded O2 at this temperature suggesting the pores become too small with potassium cations at or below this Si/Al ratio.
In U.S. Pat. No. 5,026,532, Coe and Gaffney describe a process for the preparation of chabazites with an Si/Al ratio ranging from 1.8 to 2.3. The synthesis of CHA (2.1) is described from aluminum hydroxide, silica sol, sodium hydroxide and potassium hydroxide with tetramethylammonium hydroxide as a templating agent. No mixed cation forms of these chabazite compositions were reported.
In U.S. Pat. No. 4,943,304, Coe and Gaffney teach the use of Ca exchanged chabazites with Si/Al of 1.8 to 2.7 for the purification of bulk gases from impurity gases wherein the bulk gas is largely excluded from zeolite. The bulk gases include gases that bind less strongly to the chabazite than nitrogen as well as those that are excluded because of their larger size than nitrogen. These include argon, hydrogen, helium, krypton, neon, and tetrafluoromethane among others. In this application N2 capacity data is shown for lower Si/Al calcium chabazite compositions including those with Si/Al of 1, 1.3, 1.6, and 1.72. No mixed cation chabazites in this Si/Al range are reported. Furthermore, the utility of chabazites compositions with Si/Al lower than 1.8 is dismissed, based on the relatively low nitrogen capacity of the compositions compared with chabazites with higher Si/Al ratio. P. Webley and coworkers (J. Phys. Chem. C, 2013, 117, 12841-12847) disclose a range of CHA compositions which vary in cation content and Si/Al ratio and include mixed cation type chabazites. In this work, framework Si/Al compositions range from 1.04 to well above 2.0; however to prepare CHA compositions with Si/Al=1.5 and below, Webley uses the process described by Kuznicki (J. Chem. Soc., Faraday Transactions, 1991, 87(1), 1031-1035). This process of making Al-rich CHA uses CHA (2.5) contacted with NaOH and alumina. According to Kuznicki, the compositions formed by his method with nominal Si/Al ratio between 1 and 2.5 are, in fact, mixtures of CHA (1) and CHA (2.5). Such materials are expected to give a mixture of the adsorption behavior of the two materials in the composition. A single, single phase composition is not derived by this method and so the mixed cation chabazite compositions reported by Webley, differ from the low Si compositions of this invention. The compositions of this invention are essentially of a single phase and therefore are expected to have a single pore structure and dimension, and unique adsorption properties. By “single phase” it is meant that the composition is at least 90% of one type of CHA with a specific Si/Al ratio, e.g. CHA (1.6), as opposed to blends of chabazites with two different Si/Al ratios.
Thus, by virtue of the potential benefits, there is a need for homogenous, single phase zeolite adsorbents having high kinetic selectivity for O2 over Ar and O2 over N2 adsorption at both ambient and sub-ambient temperatures. Disclosed herein are zeolite compositions, particularly chabazite compositions that meet these and other goals.
For the adsorptive separation of O2 from oxygen-containing streams using zeolitic adsorbents, the Webley group (Adsorption, 11, 173-177 (2005)) describes the use of potassium chabazite with Si/Al ratios of 2.4 to demonstrate selective adsorption of O2 over Ar at a range of temperatures from 0° to 50° C., however such equilibrium based materials have been observed to exhibit very slow uptake of O2.
A number of other researchers mention equilibrium-based separations of O2 from Ar, but there are few rapid kinetic separations reported. Most of these kinetic separations use carbon molecular sieves (CMS), which by virtue of size selectivity are able to absorb O2 at rates about 30× faster than Ar at ambient temperatures (U.S. Pat. No. 6,500,235 and S. U. Rege and R. T. Yang, Adsorption, 2000, Vol. 6, 15-22). U.S. Pat. No. 6,500,235 also mentions the use of a transition metal containing metal organic framework (MOF). Both materials show relatively slow uptake, and because they contain organic components, must be filtered should these streams be recycled back to a cryogenic stream.
Only a few zeolite materials have been reported for the kinetic separation of O2 from N2 or Ar at ambient temperatures. S. Kuznicki, B. Dunn, E Eyring and D. Hunter (Separation Science and Technology 2009, 44:7, pp 1604-1620) report the kinetic separation of O2 from Ar using the Ba exchanged titanosilicate, BaRPZ-3. Rates appear to be slow for this material and the kinetic selectivity of O2/Ar<10. S. Farooq (Gas Separations and Purification, Vol. 9, No. 3, pp 205-212) describes the possible use of a modified 4A material, RS-10 from UOP, which shows kinetic selectivity for O2 adsorption over N2 and Ar comparable to carbon molecular sieves, at similar O2 uptake rates to the CMS materials.
At sub-ambient temperatures, D. W. Breck (Zeolite Molecular Sieves, Robert E. Krieger Publishing Co., 1984) describes the use of zeolite 4A in kinetic O2/N2 and O2/Ar separations.
Kovak et al. in U.S. Pat. No. 5,159,816 mention the use of a list of zeolite adsorbents including 5A, 4A, mordenite, 13X, and chabazite for oxygen and nitrogen adsorption in cryogenic TSA processes.
Gary et al. in U.S. Pat. No. 6,083,301 teaches the use adsorbents including zeolites with a Si/Al ratio<1.15 for removing impurities from an inert fluid stream; however, the specification teaches the use of zeolite X as the zeolite adsorbent.
Thus, it is desirable to develop zeolite adsorbents that are useful in the separation of O2, N2, and Ar. Such adsorbents themselves can have any of the following properties: greater selectivity, faster adsorption and desorption, increased O2 capacity, increased productivity and easier regeneration (e.g. heat-free, or vacuum-free), and other qualities.